MRI volumes of amygdala and hippocampus in non–mentally retarded autistic adolescents and adults

Methods.

Results.

There were no significant differences between the autistic subjects and individually matched control subjects on any demographic variable or on height and weight (table 1). Table 2 presents structure volumes for the autistic and control subjects. Amygdala volume was significantly smaller for the autistic than for control subjects, both with (p = 0.006) and without (p = 0.01) correction for total brain volume. Total brain volume and absolute hippocampal volume did not significantly discriminate between groups. When corrected for total brain volume, hippocampal volume was significantly reduced in the autistic group (p = 0.04). A repeated-measures analysis of variance indicated no significant interaction between side (left versus right) and diagnosis (autism versus control) for either the hippocampus (F = 0.036, p = 0.85) or amygdala (F = 0.003, p = 0.95), indicating that degree of asymmetry did not differ between the two groups.

Because of the controversy about brain enlargement in people with autism, it might be argued that height is a better correction factor than brain volume for assessing group differences in specific brain regions. Analyses were repeated using height as a covariate. These analyses are associated with a loss of power, however, because they do not take advantage of the careful individual matching of autistic and control subjects. These analyses yielded significant group differences for amygdala (F = 11.23, p = 0.003) and a trend toward significance for hippocampus (F = 3.59, p = 0.073).

Discussion.

This study provides the first evidence of volumetric abnormalities of the amygdala and hippocampus in vivo in autism. The volume abnormalities were bilateral, with no between-group difference in the symmetry of amygdala and hippocampal volumes. A secondary finding was the absence of a significant increase in total brain volume in these autistic subjects. Because the autistic subjects were not mentally retarded, the reduced volumes of the amygdala and hippocampus are presumed to reflect and be specifically associated with the subjects’ autistic features, rather than a possible reflection of the mental retardation that commonly accompanies autism. The volume reduction was most pronounced for the amygdala, which was significantly smaller in the autistic subjects both with and without consideration of total brain volume. For the hippocampus, the reduction in volume in the autistic subjects was apparent only when hippocampal volume was considered in relation to total brain volume. The more pronounced differences in structure volumes with correction for total brain volume appeared to reflect the effect of slightly greater total brain volumes and the smaller volumes of the amygdala and hippocampus in the autistic subjects.

The reduction in the MRI volumes of the amygdala and hippocampus is consistent with the histopathologic observations by Bauman and Kemper3,9,10 of increased packing density of small, immature-appearing neurons with truncated dendritic development indicative of curtailment in the maturation of the neurons and neuropil. This histopathologic feature likewise suggests that a reduction in the neuropil of the amygdala and hippocampus, and, by inference, their connectivity with other brain regions, and not a destructive lesion, is the most likely basis for the MRI volume reductions in these structures. In a recent preliminary neuropathologic study in which a single slice of hippocampus was examined, Bailey et al.2 did not observe changes in cell packing density. Because Golgi studies were not performed by Bailey et al., dendritic development in hippocampal neurons was not assessed. Furthermore, examination of the amygdala was limited by tissue sampling for neurochemistry.

The more pronounced reduction in amygdala volume and more subtle reduction in hippocampal volume in the non–mentally retarded autistic subjects in this study are consistent with the more extensive histologic abnormalities in the amygdala and less extensive abnormalities in the hippocampus observed by Bauman and Kemper10 in one non–mentally retarded autistic subject, compared with autistic subjects who were mentally retarded. Preliminary neuropathologic examination of the brain in patients with Asperger’s syndrome, a disorder in which the deficits are of the same quality but of lesser severity than in most cases of autism, has revealed similar histologic changes in the amygdala but not the hippocampus.9 These cases of high-functioning autism and Asperger’s syndrome suggest that the amygdala and its neural connections may play a more essential role than the hippocampus in the pathophysiology of the behavioral features of autism.

The findings of the current study are not consistent with the two volumetric imaging studies of hippocampus to date in autism, which have reported negative findings,12,13 even when controlling for total brain volume.13 Differences between our findings and those of previous studies may reflect differences in samples. For example, our sample was higher functioning than previous samples because both Verbal IQ and Full Scale IQ were required to be above 80. In the study by Saitoh et al.,13 Verbal IQ or Performance IQ was required to be above 70, and in the study by Piven et al.,13 Performance IQ was required to be above 70; these IQ criteria typically result in autistic groups that are more severely affected. Methodologic differences between the studies might also be responsible for differences in findings. Saitoh et al.12 performed measurements on only three 5-mm cross-sectional slices through the midsection of the hippocampus, and no attempt was made to match subjects and controls on IQ. In the study by Piven et al.,13 methodology for the volumetric measurements was similar to that used in the current study, although the image analysis program used in the current study allows the rater to trace the borders (not just visualize structures) simultaneously in the three planes. The autistic and control groups in the study by Piven et al.13 were not matched on age, sex, or IQ, variables known to have a significant influence on brain morphology, although these variables were used as covariates at the time of statistical analysis to control for the effect of group differences. The detection of volume differences in the current study may reflect the combined use of true volume measurements, more accurate tracing of margins, and more stringent control of confounding variables by individual matching at subject selection, as well as sample differences.

The amygdala and hippocampus have long been thought to play a role in the pathophysiology of this syndrome, and the findings of the current study provide support for the involvement of these structures. The function of the amygdala has most often been considered in relation to its role in the evaluation of emotional stimuli20 and the impact of emotional intensity, particularly fear, on memory and learning. Recent functional MRI and PET studies21,22 have demonstrated increased activity in the amygdala when normal subjects viewed happy as well as fearful facial expressions compared with emotionally neutral faces. Animal studies have emphasized the impact of early amygdala damage on social behavior in monkeys, with lesions during infancy producing lack of interest and ineptness in social contact as well as lack of facial expression.23 Early neurobehavioral models proposed that the abnormal social behavior in autism was the result of the failure of the amygdala to attach emotional valence to people, reflecting observations of the absence of social behavior and affect in severely autistic people. However, subsequent research demonstrated that the social and affective deficits each were deficits in their own right.24 Investigation of the social deficit demonstrated that a substantial proportion of autistic people develop attachments to people and experience basic emotional states.24 Major deficits were documented in social cognition, particularly in the capacity for making inferences about the beliefs, desires, emotions, and intentions of other people (“theory of mind” deficits).25 These deficits in social cognition are now widely recognized as playing the predominant role in the social manifestations of autism. The cognitive nature of these deficits has suggested that the social behavior is most likely related to the underdevelopment of extensive neural systems that prominently involve cerebral cortex, rather than localized dysfunction of amygdala. This localization is supported by a PET study of brain activation during a “theory of mind” task, reporting that subjects with Asperger’s syndrome did not show the normal activation of the medial part of the left prefrontal area, but instead showed significant activation of a neighboring area, suggesting that they were using a more “general-purpose” reasoning mechanism to infer mental states.26

Investigation of affect and emotion in autism has demonstrated a reduced capacity for exhibiting and comprehending emotions in facial expressions and prosody, for experiencing emotions, and for cognitively defining more abstract emotions.24,27 Less impaired autistic people develop competency with basic emotional states, but lack comprehension and expression abilities for the more subtle emotional shadings and for the cognitive concepts of more abstract emotions.24,27 These higher-order deficits in emotion, affect, and emotional cognition likewise suggest that the neurobiological basis of the emotional and affective deficits in autism involves not only the amygdala but more extensive neural systems and connections with other brain regions, particularly cerebral cortex. The histopathologic features of the amygdala in autism also provide evidence that it is not localized destruction of the amygdala that underlies dysfunction, but underdevelopment of what is likely to be extensive neural connections and neural systems that subserve social and emotional capacities.

The hippocampus is best known for its role in associative memory and in amnesia, and an amnesic disorder was hypothesized as the cause of autism for several decades. However, the neuropsychological evidence supporting the analogy to amnesia was limited to a single study,28 and the memory deficits reported were likely a reflection of the failure to match autistic and control subjects on ability. Multiple subsequent studies of memory using the more specific current diagnostic criteria for autism and screening for other disorders have consistently demonstrated the integrity of associative memory processes, including immediate recall and recognition memory.29,30 Such studies have instead found evidence of reduced use of the cognitive organizing strategies that support memory, deficits more consistent with dysfunction in neocortical memory systems than with localized hippocampal dysfunction. This pattern of preserved associative memory and impaired metamemory function is consistent with the histopathologic evidence indicative of reduced neural connections between hippocampus and other brain regions.

Results of the current study also revealed no significant differences in brain volume between autistic and control subjects. One prior MRI study reported a significant increase in brain volume in autistic subjects of the same age but lower IQ than the subjects in this study.31 Neuropathologic studies2,9,10 have reported increased brain weight in a subgroup of cases, and megaloencephaly has been reported in 2% to 42% of subjects with autism,32 with greater incidence among children than among adults or neonates.33,34 The influence of age and IQ, a commonly used index of autism severity, is unknown. The failure of the current study to document an increase in brain volume may reflect the higher level of functioning among our subjects, the small sample size, or the occurrence of increased brain weight and volume in only a subgroup of autistic people.

The methodologic advantages of this study include the rigorous control of age, IQ, and sex effects through individual matching and prospectively selected volunteers from the community who were screened for both personal and family history of neuropsychiatric disorders. Methodologic improvements in measurement of the hippocampus and amygdala were facilitated by the acquisition of MRI scans with 1.5-mm slice thickness and the capability for viewing and outlining structures simultaneously in multiple imaging planes. The major limitations of the study were the relatively small sample size and the absence of measurements of other structures known to be functionally related to amygdala and hippocampus. Additional imaging studies are needed in autism to establish large sample sizes that document the influence of age and IQ and allow correlations between brain structures thought to be functionally connected to hippocampus and amygdala.